1. Field of the Invention
This invention relates generally to a chemical shaving and pickling process for use in the manufacture of cast copper rod. More particularly, it relates to a process for chemical shaving and pickling employing a combination of sulfuric acid and hydrogen peroxide operated under controlled time, temperature, and concentration conditions so as to provide an improved surface substantially free of surface oxides for continuously cast copper rod destined for subsequent wire drawing or rolling operations.
2. Discussion Of The Prior Art
For many years, copper rod which is intended to be drawn into wire has been prepared by statically or continuously casting electrolytically refined copper into cast wire bar or continuously cast bar, respectively; conditioning the cast bar for rolling; rolling the cast bar in a hot-reversing or multiple stand hot mill to an intermediate size; further rolling in a multiple-pass rolling process to a desired hot-rolled size; and coiling the hot-rolled rod. As a result of the casting, conditioning, and hot-rolling operations, oxide scale forms on the surface of the rod. The scale may include cuprous oxide (Cu.sub.2 O)and cupric oxide (CuO). In order to remove the scale in preparation for the subsequent drawing or rolling process, it is necessary to add a pickling step following hot rolling. While the pickling may be performed in a batch process by dipping the coiled rod in an appropriate solution for a predetermined time period, it is also possible, and frequently desirable, to employ a continuous pickling process, such as the so-called "Dr. Otto" pickling system, wherein loops of copper rod from the coil are passed sequentially through pickling, rinsing, and coating tanks on a continuous conveyor. A number of pickling solutions have been used in the past including aqueous solutions containing sodium bichromate, hydrogen peroxide, and sulfuric or other acids or combinations thereof. Sodium bichromate and hydrogen peroxide are oxidizers while sulfuric acid and other acids function as reducing agents. Sodium bichromate is not a practical pickling agent for copper because the copper with which it reacts cannot easily be recovered and the products of reaction cause pollution problems. Where sulfuric acid (H.sub.2 SO.sub.4) is used as the active agent in an aqueous pickling solution, commonly at a concentration of about 20% and a temperature of about 120.degree.-180.degree. F. the principal reaction is: EQU CuO +H.sub.2 SO.sub.4 .fwdarw.CuSO.sub.4 (aq.)+H.sub.2 O, (1)
though the following reaction also occurs to a limited degree: EQU Cu.sub.2 O+H.sub.2 SO.sub.4 .fwdarw.CuSO.sub.4 (aq.)+H.sub.2 O+Cu. (2)
The second reaction, involving cuprous oxide (Cu.sub.2 O), produces a red copper powder which should be completely removed from the rod before drawing commences. Experience with this reaction has shown, however, that sulfuric acid alone does not remove all of the red cuprous oxide and that, over time, some of the remaining red cuprous oxide further oxidizes to a black cupric oxide.
In order to improve the effectiveness of sulfuric acid pickling, hydrogen peroxide (H.sub.2 O.sub.2) has been added to the aqueous pickling solution since hydrogen peroxide is effective to oxidize cuprous oxide (Cu.sub.2 O) to cupric oxide (CuO) according to the following reaction: EQU Cu.sub.2 O+H.sub.2 O.sub.2 .fwdarw.2CuO+H.sub.2 O. (3)
Hydrogen peroxide can also react with metallic copper when the oxide coating has been removed but, as noted below, this reaction does not occur under ordinary acid pickling conditions. Moreover, hydrogen peroxide is relatively unstable and must be stabilized in order to provide reliable process results.
In addition to pickling processes, the art has also practiced mechanical shaving of copper rod in order to produce a clean and uniform surface. However, while mechanical shaving is effective, it is necessary to remove relatively large quantities of copper from the rod surface which increases the copper loss and the processing expense. As a result, mechanically shaved rod is a premium product which is commercially acceptable only in limited areas.
As an alternative to the usual pickling process, producers of copper rod have employed sulfuric acid cooling procedures wherein the hot-rolled copper rod is cooled with sulfuric acid between the rolling and coiling steps and then washed in order to remove the resulting red copper powder. In this process, the mechanical effect of the water and the difference between the coefficients of expansion of the copper and copper oxides help to separate mechanically the oxides from the base copper. In some cases, alcohol is substituted for sulfuric acid as a coolant and reducing agent.
Despite the various pickling, cleaning, and conditioning processes that have been developed heretofore, an economically satisfactory process for removing surface oxides and for providing an improved surface for copper rod intended for wire drawing has not been available.